Conventional grid-tie inverters utilize a current-mode control approach that injects an in-phase alternative current (AC) of a sinusoidal waveform into the utility grid. This has been accomplished without active power factor correction with respect to a nearby load, which shares the same grid voltage and the utility power transmission line. In general, the grid impedance with the long transmission line does not preserve an ideal voltage source characteristic, exhibiting a reactive impedance component such as some inductive component with some resistive component. Consequently, the almost ideal voltage source characteristics of the utility grid may not always exist and the current-mode control approach used to actively achieve the sinusoidal waveform of the inverter output current may not be possible.
Some distortion of the inverter output current may not be avoidable using the current-mode control approach, because the driving voltage within the inverter power stage has a very limited magnitude. This limited magnitude prohibits its ability to fully realize an ideal sinusoidal current-source waveform. Alternatively, the inverter power stage can be controlled to be a voltage source, allowing the inverter output current to become a natural response of the inverter and the utility grid. The inverter and the utility grid are interconnected to co-dependently contribute to the system response. Practically, the inverter output voltage of a sinusoidal waveform is much easier to achieve as compared to the inverter output current, which is a dependent response.
Thus, a voltage-mode grid-tie inverter that delivers its sinusoidal output current as a natural response of the interconnected inverter and utility grid may be beneficial.
Despite the variety of load types (e.g., resistive, capacitive, or inductive) that are terminated across the inverter output, both the wave-shape and the phase of the inverter output voltage are independently controllable with respect to the grid voltage. The grid current is simply the natural response, which can be managed to be either in phase or out of phase with the grid voltage, i.e., active power factor correction. The conventional current-source inverter requires a sufficient control bandwidth to regulate its output current waveform to be sinusoidal, leading to difficulties to achieve stable control due to the uncontrollable interactions existing between the back-end electromagnetic interference (EMI) filter's resonance and the inverter current-source control loop. To avoid the difficult issues of instability associated with the conventional current-source inverter, it may be beneficial to have a voltage-source inverter that includes a control loop designed for stable operation even with a non-ideal utility grid, which may possess far from ideal characteristics of a voltage source.